One explanation for this would be the presence of two

different tyrosyl-tRNA synthetases, one of which would be induced under stress conditions (acidic pH and extremely low tyrosine concentration). In general, there is only one aminoacyl-tRNA-synthetase for each amino acid in most bacteria, however, several exceptions are known. Indeed, two see more very similar lysyl-tRNA synthetases, lysS (constitutive) and lysU (heat inducible) have been described in Escherichia coli [29]. In gram-positives, in addition to the aforementioned case of the two tyrS of E. faecalis, there are two distinct histidyl-tRNA synthetase genes in Lactococcus lactis [30], and two tyrosyl-tRNA synthetase genes (tyrS and tyrZ) and two threonyl-tRNA synthetase genes (thrS and thrZ) in Bacillus subtilis [31, 32]. In this last case, the normally silent thrZ gene is induced during threonine starvation or by reducing the intracellular concentration of ThrS, which is the housekeeping threonyl-tRNA synthetase sufficient for normal cell growth [33]. The location of genes encoding an aminoacyl-tRNA-synthetase associated to the gene clusters involved in tyramine and histamine

biosynthesis is a general feature [9, 10, 14, 16–18, 34]. One of the reasons to study the expression of tyrS A-1210477 in E. durans is to find out whether this protein could have a role on the genetic regulation of the tyramine cluster, being activated under limiting Florfenicol levels of tyrosine to prevent massive decarboxylation of this amino acid, ensuring its availability for protein synthesis. Consistent with this idea would be 1) the common location of genes encoding aminoacyl-tRNA sinthetases next to the operon of decarboxylation (BA-biosynthesis) of the corresponding aminoacid [9, 10, 14, 16–18, 34], 2) the expression of this gene only under acidic pH, which is the condition

regulating positively the biosynthesis and accumulation of tyramine [19, 35] and 3) the fact that tyrS and the genes of the tyramine biosynthesis pathway (tdcA and tyrP) require opposite conditions of tyrosine concentration for optimal expression (Figure 5) [19]. Altogether, these data raise the question whether TyrS could act as a negative regulator. However, overexpression of tyrS on multicopy plasmid during growth of the E. durans strain carrying the wild-type allele had no observable effect on the expression profile of the decarboxylating gene tdcA or on the tyramine concentration observed in the supernatant. Figure 5 Genetic organization and transcriptional profile of the TDC cluster in E. durans IPLA655. Promoters (P) and termination regions are indicated. The different mRNA are represented by wavy lines. Numbers indicate the size of the corresponding gene in base pairs (bp). Regulation of the genes by tyrosine and pH is indicated below. Acidic pH is required for optimal expression of the three genes.